1. Field of the Invention
The present invention concerns an imaging tomography apparatus and a method for operating such an apparatus, in particular an x-ray computed tomography apparatus, of the type having at least a first acquisition system, with a first radiator and a first data acquisition unit for detection of radiation originating from the first radiator, and a second acquisition system, with a second radiator and a second data acquisition unit for detection of radiation originating from the second radiator, wherein both acquisition systems are capable of rotating around a common rotation axis with a constant angular separation in the azimuthal direction.
2. Description of the Prior Art
Tomography apparatuses of the above type are known, for example, from U.S. Pat. Nos. 4,991,190, 4,384,359, 4,196,352, 5,966,422, and 6,421,412. An advantage that such tomography apparatuses with a multiple acquisition systems exhibit in comparison to an apparatus with one acquisition system is an increased data acquisition rate, which leads to a lower exposure time, and/or to an increased temporal resolution. A shortened exposure time is of advantage because with it, movement artifacts in the reconstructed image (for example, caused by voluntary and involuntary movements of the patient and/or by arrhythmias in the heart movement) are minimized. This is particularly of importance in the event a larger volume is scanned, for example by means of a spiral scan, for example of the heart. An increased temporal resolution is, for example, necessary for representation of movement cycles, because then the data used for reconstruction of an image must be acquired in the shortest possible time. Conventionally, this has been attempted to be achieved by increasing the rotation speed of the acquisition system, however the acceleration forces, and the mechanical problems resulting therefrom, increase significantly with additional rotation speed. Such problems can be solved with the cited tomography apparatuses, which have multiple acquisition systems (radiator-detector combinations) arranged separated from one another in the azimuthal direction, meaning angularly offset relative to one another. This type of tomography apparatus is particularly advantageous when spiral reconstruction algorithms are used for reconstruction of images from the raw data generated by the detectors, which only require projection data from an angular interval of 180°, because then, for example given the presence of two acquisition systems, the exposure time is reduced to a quarter of the measurement time required for a full rotation.
In x-ray computed tomography, unwanted artifacts can occur in the CT image if scattered x-ray quanta also reach the detector in addition to the primary quanta, due to x-ray quanta scattered in the examination subject. This is generally counteracted by scattered-ray collimators or scattered-ray grids that are positioned directly in front of the detector. The grids, fashioned from absorbing materials, have fine and suitably aligned channels that allow only the non-deflected (and thus image-relevant) x-ray quanta reach the detector, By enlarging the collimator height by an additional depth of the channels, the scattered-ray suppression could be improved, however this is at the cost of efficiency. Additionally, such an increase of the collimator height would be very difficult mechanically to control, and would in addition increase the costs for the detector system.
Numerous methods for scattered-ray correction are known, for example from the professional article by B. Ohnesorge, T. Flohr, K. Klingenbeck-Regn with the title “Efficient object scatter correction algorithm for third and fourth generation CT scanners”, Eur. Radiol. 9, pages 563–569, 1999. Using a convolution model, the portion of the image intensity tracing back to the scatter radiation is determined, and is subtracted from the measured signal, and thus a signal largely traced back to the primary quanta is obtained.